Micrometer-sized spherical particles of bismuth tungstate (Bi2WO6) with hierarchical architecture were prepared by facile hydrothermal reaction without using any surfactants and polymers as structure-directing agents. The particles were assemblies of polycrystalline flakes composed of rectangular platelets. The hierarchical polycrystalline particles of "flake-ball" shape exhibited relatively high photocatalytic activity for oxidative decomposition of acetic acid in aqueous suspensions.